1. Field of the Invention
This invention relates to a method in which a part of an elongate substrate is covered by overlappingly wrapping a heat-shrinkable polymeric sheet or other similar closure member concentrically around the substrate, securing the overlapping edge and the underlying overlapped portion of the sheet together with a patch closure and heating the thereby formed, essentially tubular closure member so that it shrinks into circumferential contact witht the substrate.
2. Summary of the Prior Art
Many methods of the kind outlined above are known in the art, and reference may be made, for example, to U.S. Pat. Nos. 3,379,218; 3,455,336; 3,530,898; 3,542,077; 3,574,313; 3,770,556 and U.S. Pat. No. 4,200,676, the disclosures of which are incorporated by reference herein. U.S. Pat. No. 3,770,556 discloses a method in which contact adhesive is provided on opposing surfaces of a wrap-around closure member which opposing surfaces contact each other in the wrapped configuration, and a cover sheet, e.g. aluminum foil, fabric tape or heat-shrinkable plastic, is stuck over the exposed outer edge of the closure member to prevent peeling back thereof during the heat-shrinking of the closure. U.S. Pat. No. 4,200,676 discloses a method in which the wrapped edges of the closure member are secured together by means of a multi-layer cover sheet which comprises a polymeric layer and a reinforcing member and which cover sheet has a coating on its inner surface of a cross-linked hot-melt adhesive. The cover sheet is placed over the exposed outer edge of the closure member and is then heated and pressed against the closure member to secure the edges thereof together prior to heat-shrinking of the closure member.
While the methods of the prior art outlined above are very useful in a number of situations, all of them suffer from one or more disadvantages, such as the need for specially formed edge portions on the closure member, or the inability to form a consistently reliable closure when the closure member has a high recovery force, or when the substrate has an irregular cross-section (e.g. a transition in a pipe system), or when the substrate diameter is small. In particular, the method of U.S. Pat. No. 3,770,556 poses a problem where a wide range of sizes of substrate must be covered. Since both opposing surfaces of the closure member must be coated with contact adhesive the diameter range of the formed closure is perforce limited. Although such a closure member can, merely because it is heat recoverable, accommodate some variation in the substrate diameter, use of a highly expanded material for the closure member imposes recovery forces beyond that which contact adhesives can generally withstand. In addition, contact adhesives almost invariably have poor peel strength in comparison with hot-melt adhesives. Therefore a closure of the type taught by U.S. Pat. No. 3,770,556 would be inappropriate where the closure would be subjected to significant peel forces.
The patch design of U.S. Pat. No. 4,200,676 presents certain difficulties also. First, this patch is of relatively complex and bulky construction and therefore heats up and cools down relatively slowly and further tends to insulate the underlying adhesive layer. In addition, a major shortcoming of the patch design of U.S. Pat. No. 4,200,676 exists in connection with covering substrates of irregular diameter as in the case where a telephone or high-voltage cable transition or a joint between pipes of different diameter is to be covered. To accommodate such a transition the patch must be able to deform (i.e. stretch) in the areas of greatest stress to thereby more evenly distribute the recovery stress and avoid peeling of the patch away from the closure member. The substantially undeformable patch of this prior art cannot stretch significantly and hence is subject to pulling away of the closure in regions of high recovery stress such as is caused by a major diametric transition. Likewise, a reinforced patch is unsuitable for substrates of small diameter since its comparative stiffness makes it difficult or impossible for the installer to conformably wrap it around the closure member which, being flexible, does readily conform to the substrate.
Wallace, U.S. Applications Ser. No. 58,743, filed July 19, 1979 and Ser. No. 172,394, filed July 25, 1980, both now abandoned, the combined disclosure of which has been published as European Patent Application Publication No. 23788 and is incorporated herein by reference, discloses an alternative method wherein the patch is capable of extension in the circumferential direction of the closure. As with U.S. Pat. No. 4,200,676, the adhesive is a cross-linked hot-melt adhesive.
In the case of both U.S. Pat. No. 4,200,676 and European Patent Application No. 23788, the patch is required to be sealed completely to the closure member before the combined closure is heat recovered; and, in addition, a hot-melt adhesive of high viscosity is required. This increased viscosity is usually produced by cross-linking the adhesive, increasing the cost of the product. Further, although both the above documents suggest that a pressure-sensitive adhesive may be used in combination with the patch, they regard such a use as merely an initial hold-down means for the patch before activation of the hot-melt adhesive, and consider it disadvantageous for its reduction of the total strength of the bond between the patch and closure member, and regard other hold-down means as preferable.